The present invention relates to scintillation detectors for measuring gamma ray adiation and more particularly to an improved form of scintillation detector having a unique construction which provides exceptional shock resistance while maintaining high detection efficiency.
Thallium activated sodium iodide crystals are very effective gamma ray detectors. Such crystals have for many years been used in scintillation detectors which are employed in the oil industry for deep well logging. These detectors are lowered into deep bore holes in special tools having relatively small diameters. Their ability to detect gamma rays makes it possible to analyze surrounding rock strata. When they are lowered to different depths they provide an excellent means for analysis of geological formations at each test bore hole.
For many years sodium iodide crystals for deep bore hole well logging have been enclosed or encapsulated in metal tubes having relatively small diameters. An optical window is provided to close one end of the tube, and a thin optical coupling is provided between the sodium iodide crystal and the window. The crystal has a diameter less than the internal diameter of the tube and is surrounded by a highly reflective material, such as aluminum powder. In a conventional scintillation detector, a spring assembly is provided at the end of the metal tube opposite the optical window to bias the crystal toward the optical coupling at the window.
In conventional assemblies where the optical coupling is a fluid, such as a heavy silicone oil, an O-ring is placed around the crystal near the window to provide a seal. Another O-ring is placed at the opposite end of the crystal to prevent escape of the reflector material and to help center the crystal in the tube. Sodium iodide crystals are brittle, weak in tensile strength, have no elasticity, and therefore have little shock resistance. Consequently, prior to this invention, the shock resistance of scintillation detectors was limited, and they could be broken or seriously damaged, for example by shocks of 50 g's or more. They were unsatisfactory for use where high shocks were to be encountered such as 100 g's or more.
For this reason, prior designs were not satisfactory for larger scintillation detectors employing sodium iodide crystals of substantial weight. For example, scintillation detectors of conventional construction did not have adequate shock resistance if the crystal had a substantial diameter, such as 1.5 inch or more, and a substantial length, such as 12 inches or more.
If a conventional scintillation detector of such size were used for deep well logging, the expected useful life would be relatively short because of the likelihood of premature shock damage. For example, 100-g shock load perpendicular to the axis of the steel cylinder could cause the sodium iodide crystal to bend between the O-rings at opposite ends of the crystal and to fracture the crystal. A 100-g shock load in the direction of the axis away from the optical window could cause the sodium iodide crystal to compress the spring a substantial amount and thereby destroy the optical coupling between the crystal and the glass window. If the crystal were bonded so well that it did not break the optical coupling, the g forces would be likely to exceed the tensile strength of the crystal so that is could be torn apart.
If a 100-g shock load is applied to a conventional detector in the opposite direction along the axis of the crystal toward the glass window, the crystal and/or the glass would be subject to fracture because the optical coupling provides no protection.
Thus, in scintillation detectors of the type known prior to this invention, there was no satisfactory solution to the shock problem when the sodium iodide crystals were large and the shock loads approached 100 g's. The problem could not be solved by increasing the strength of the spring as the crystal size increased because this would result in too much pressure on the glass window and would transmit excessive shocks to the crsytal.